Apparatus for passing stock into a headbox of a paper machine or equivalent

ABSTRACT

A vacuum pump (P 0 ) provides vacuum to a paper machine deaeration tank ( 11 ) which has an inlet duct ( 10 ) through which the tank is supplied with wire water. A discharge duct ( 12 ) extends from the deaeration tank ( 11 ). There is an overflow ( 14 ) for the wire water in the discharge duct at the discharge end of the discharge duct ( 12 ). The overflow ( 14 ) of the discharge duct ( 12 ) is located below the deaeration tank ( 11 ) and opens to a free air space. The discharge duct ( 12 ) includes a branch duct ( 15   a   1 ) for a flow which is passed to the headbox ( 100 ).

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/F101/01129 and claims priority on FinnishApplication No. 20002896, filed Dec. 29, 2000, the disclosure of whichis incorporated by reference herein.

STATEMENTS AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for passing stock to a headbox ofa paper machine or equivalent.

SUMMARY OF THE INVENTION

When a deaeration tank is situated between centrifugal cleaning and aheadbox feed pump in the short circulation, it should act not only as ameans of removing air but also as a pressure equalizer before theheadbox feed pump. However, the deaeration tank does not guarantee aconstant pressure since, in the pressure conditions of the deaerationtank, air bubbles take a large part of its liquid volume. For example,5% of air takes 50% of the liquid volume at a pressure of 0.1 atm. Thefoaming of air bubbles in the deaeration tank is unstable. The overflowfrom the deaeration tank evens out the surface but it pays no attentionto whether there is air or gas in the liquid space of the deaerationtank. For this reason, the pressure after the deaeration tank varies.The pressure variations are transmitted to the headbox. The pressurecontrol of the headbox attempts to maintain a uniform pressure in theapproach pipe, but it affects the level of the surface in the deaerationtank, intensifying the disturbance. Mere removal of the overflow fromthe deaeration tank is not enough because the pressure variation turnsinto consistency variation.

In accordance with the invention, the deaeration tank can beaccomplished without an overflow if it is in hydraulic connection withan overflow surface in the cellar. For example, the lock water part in acirculation water tank can act as an overflow surface if the deaerationtank is arranged to treat wire water in the short circulation. It isadvisable to build the overflow so that it is wide. When shortcirculation dilutions, among other things, for the headbox feed and fanpumps are taken from a connecting pipe, the pressure of the shortcirculation remains constant, because it is determined by a stableoverflow at normal pressure, in which there is no problem caused bybubbling of air. In controlling the vacuum in the deaeration tank, turboblowers may be more suitable than vacuum pumps. The process is simplerthan before, consumes less energy and requires a considerably smallerprocess volume. When the flow of wire water increases, the speeds in thecyclone and in the spray tubes of the deaeration tank increase, whichincreases deaeration capacity.

Thus, in accordance with the invention, the deaeration tank is providedwith an overflow which is disposed at the distance of the static heightdifference of the liquid column required by vacuum from the deaerationtank, for example, in the cellar space in the paper machine hall. Theheight difference between the liquid surface of the deaeration tank andthe overflow surface is advantageously in a range 5 to 10 m.

The back-pressure required by the pump is about 3-4 m counted as astatic water column depending on the pump. The level of the overflowsurface need not be controlled, i.e. it may be fixed, so that it isabout 8 m below the surface of the liquid in the deaeration tank (8 mcorresponds to a vacuum of 80 kPa in the deaeration tank). In thatconnection, it must be possible to adjust the vacuum level in thedeaeration tank. The deaeration tank would then be located about 13 mabove the cellar level, i.e. about 4 m above the machine level. Thepressure loss in the spray feeding of the deaeration tank is about 3-5m. When the pressure loss of the deaeration tank is added to theback-pressure of the pump, a level slightly lower than the machine levelis achieved, which means that the waters coming from the wire can betreated with the system in question without additional pumping. Thediameter of the distributor pipe is in a range of 0.3 to 2.0 m and thetime in which the liquid flows from the deaeration tank to the biggestsite of use is less than 2 minutes, in practice about 5 seconds. Thus,the invention employs a deaeration tank which has no overflow, while, inaccordance with the invention, the discharge duct of the deaeration tankcomprises an overflow. The overflow is preferably disposed in a wirewater tank or equivalent. In accordance with the invention, a branchduct/branch ducts is/are arranged to lead from the duct between theoverflow of said discharge duct and the deaeration tank to a headbox,preferably to the suction side of a headbox feed pump.

The apparatus according to the invention provides a stock flow, which isuniform both in pressure and in consistency, to the suction side of theheadbox feed pump and further to the headbox.

The paper machine or equivalent is understood to mean printing paper,board, and soft tissue machines.

In the following, the invention will be described with reference to someadvantageous embodiments of the invention illustrated in the figures ofthe appended drawings, to which the invention is, however, not meant tobe exclusively confined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a first advantageous embodiment of the invention. In theembodiment of the figure, a discharge duct of a deaeration tank isarranged to lead into connection with a tank such that the end of thedischarge duct opens centrally in the tank and forms an overflow.

FIG. 1B shows an embodiment of the invention comprising a discharge ductwhich is connected directly to a tank which comprises an overflow.

FIG. 1C shows an embodiment of the invention including a tank whichcomprises two overflows, i.e. an overflow for a discharge duct and, inaddition, an overflow for the surface level of the stock in the tank.

FIG. 1D shows an additional embodiment of the invention.

FIG. 1E is an axonometric illustration of the short circulation of stockin accordance with the invention in connection with a headbox of a papermachine, as well as of a centrifugal cleaning plant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows a first advantageous embodiment of the invention whichrelates to a stock feed system of a headbox in a paper machine orequivalent and to its short circulation. Wire water is passed along aduct 10 into a deaeration tank 11, which has a discharge duct 12 forwire water from which air has been removed, and which discharge duct 12comprises at its end an overflow 14, in which connection the heightdifference H₁ between the level T₂ of the overflow surface 14′ of saidoverflow and the surface T₁ of the wire water in the deaeration tank 11is in a range of 5 to 10 m. The deaeration tank 11 comprises for wirewater an inside tank space D, which comprises a vacuum space D′ abovethe liquid, into which space vacuum is drawn through a duct f by meansof a vacuum pump P₀, an exhaust pump or another device and, at the sametime, air is removed from the wire water. By the wire water is meantwater that is removed from a paper web on the paper machine.

As shown in FIG. 1A, the discharge duct 12 is arranged to lead intoconnection with a tank 13 through its wall and upwards in the tank. Thedischarge duct comprises the overflow 14 which is located centrally inthe tank for example, in a wire water tank 13. Branch ducts 15 a ₃, 15 a₂, 15 a ₁ open into the discharge duct 12 of the deaeration tank, wirewater and thick stock m being passed through the duct 15 a ₂ of saidducts first to the suction side of a feed pump P₂ and further to acentrifugal cleaning plant 200, from which the stock mixture is passedthrough a duct 15 a ₂′ to a mixing point of the wire water passed fromthe branch duct 15 a ₁ of the discharge duct 12, at which mixing pointthe stock mixture is diluted to a headbox consistency and passed to thesuction side of a headbox feed pump P₁ and further to an inlet header Jof a headbox 100. Dilution water is passed through the branch duct 15 a₃ to the thickness profiling system of the headbox of the paper machine.Advantageously, the connection points of the branch ducts are located inthe discharge duct 12 close to the overflow 14 and, in the heightdirection, below the overflow 14.

The discharge duct 12 can thus in itself comprise an overflow 14, i.e.the lower end 12′ of the discharge duct 12 is placed, as shown in FIG.1A, centrally inside the wire water tank 13 and said end opens upwardsto normal atmospheric pressure in the tank 13, in which connection aconstant pressure prevails in the discharge duct 12. The end result,then, is the same as in the preceding case, i.e. a constant pressureprevails in the discharge duct 12. Wire water/stock is passed by meansof a pump P₄ to the deaeration tank 11. Wire water is passed through aduct 17 of the tank 13.

FIG. 1B shows an embodiment in which a discharge duct 12 of a deaerationtank is connected to the tank 13 itself, which comprises an overflow 14.The overflow 14 is formed such that the discharge duct 12 opens into thetank 13 from its side surface 13′, in which case the tank 13 itselfcomprises the overflow 14 in its connection. The height differencebetween the overflow 14 and the surface level T₁ of the stock in thedeaeration tank is designated by H₁ in the figure. H₁ is in a range of 5to 10 m.

FIG. 1C shows an embodiment in which a tank 13 comprises two overflows,i.e. an overflow 14 of a discharge duct 12 and, in addition, an overflow140 which regulates the surface level of the stock in the tank 13 and,thus, the pressure prevailing in the tank. Wire water is circulated fromthe tank 13 by means of a pump P₄ along a duct 10 to a deaeration tank11, and wire water from which air has been removed is passed along thedischarge duct 12 into the tank 13. The discharge duct 12 of thedeaeration tank 11 is provided with a branch duct 15 a, for a headboxfeed pump P₁ which stock is passed further to an inlet header J of aheadbox 100, and with a branch duct 15 a ₂ for a feed Pump P₂, fromwhich stock is passed to a centrifugal cleaning plant 200 and further tothe headbox 100 through a passage to the branch duct 15 a ₁.

FIG. 1D shows an embodiment of the invention in which an overflow 14 ofa duct 12 is formed such that the discharge duct 12 opens into a tank 13from its side surface. The tank 13 comprises the overflow 14 for theliquid surface. Wire water from which air has been removed is passedthrough the discharge duct 12 to the tank 13. Wire water is pumped bymeans of a pump P₄ along a duct 10 into a deaeration tank 11, which islocated in an elevated position with respect to the tank 13. Thedischarge duct 12 comprises, in the vicinity of the overflow 14 but at alower level, branch ducts 15 a ₁, 15 a ₂, which comprise pumps P₁, P₂,in which connection stock is passed through the branch duct 15 a ₁ to aheadbox and through the branch duct 15 a ₂ to a centrifugal cleaningplant 200 and further to the branch duct 15 a ₁. Virgin stock m is addedto the branch duct 15 a ₂. Wire water is passed to the upper part of thetank 13 through a duct 17.

FIG. 1E shows an embodiment of the invention in which the wire waterremoved from a wire section 300 of a paper machine or equivalent ispassed along a duct 10 into a cyclone 25 and further into a deaerationtank 11. The purpose of the cyclone 25 is to remove air from wire wateralready before the deaeration tank 11 proper. The deaeration tank 11having no overflow comprises a discharge duct 12 for air-free wirewater, which discharge duct 12 ends in an overflow 14 at a lower levelin a tank 13 in accordance with the invention. As axonometrically shownin the figure, the end of the discharge duct 12 is arranged to leadthrough a side wall 13′ of the tank 13, and the end of the dischargeduct 12 opens to a free air space and a normal air pressure inside thetank 13. Thus, in the duct 12, a constant pressure prevails in theair-free wire water. As illustrated in the figure, tubes 15 a ₃ or ductsopen from the branch duct 12 in a lower position in the vertical planewith respect to the overflow 14, through which tubes or ducts dilutionwater is passed, for example, to serve as headbox dilution water or asstock dilution water. In addition, as illustrated in the figure, thickstock m is passed through a duct 17 a ₃ to the discharge duct 12 and,through a branch duct 15 a ₂ situated on the opposite side, a mixture ofwire water and thick stock m is passed from the duct 12 to the dischargeduct 15 a ₂ and further to a feed pump P₂, which feeds stock to acentrifugal cleaning plant 200. The mixture of wire water and stock ispassed from the centrifugal cleaning plant 200 to a branch duct 15 a ₂′,which is connected with a branch duct 15 a ₁ branching from thedischarge duct 12. The combined flow of stock and wire water is passedalong the duct 15 a, to the suction side of a headbox feed pump P₁ andfurther through a machine screen 18 to a stock inlet header J of aheadbox 100 of the paper machine. Additives/fillers or dilution liquidcan be added through ducts 17 a ₁ and 17 a ₂ to the stock to be fed tothe headbox. A duct 16 opens from the lower part of the tank 13 forpassing wire water to the long circulation.

A duct f leads to the deaeration tank 11 from a vacuum pump P₀ orequivalent, by means of which vacuum is drawn into the interior space ofthe tank 11 and air released from wire water is removed from the tankspace D′. In the deaeration tank air is removed only from wire water.After that, the wire water is used at different locations, among otherthings, for dilution of stock. A cyclone-shaped device 25 is used beforethe deaeration tank, a centrifugal field being produced in said deviceto separate air in the form of bubbles, and there may be severalcyclone-shaped devices 25 for different water fractions of the wiresection. A flow duct is also used before the deaeration tank, the flowcontaining more air being separated from the upper part of said flowduct.

1. An apparatus for passing stock to a headbox of a paper machine,comprising: a deaeration tank, the deaeration tank when filled with wirewater having a defined wire water surface within the deaeration tank; avacuum pump connected to the deaeration tank to pull a vacuum on thedeaeration tank; an inlet duct connected to the deaeration tank and toat least one source of wire water, so that wire water can flow from theat least one source of wire water to the deaeration tank; a dischargeduct connected to the deaeration tank, the discharge duct having adischarge end, the discharge end having an overflow, the overflowdefining an overflow surface, said overflow of the discharge duct andthe overflow surface defined thereby being located below the definedwire water surface within the deaeration tank and opening to a free airspace; and wherein the discharge duct includes a first branch ductextending from the discharge duct and in communication with the headbox.2. The apparatus of claim 1 wherein the end of the discharge duct formsthe overflow.
 3. The apparatus of claim 2 wherein the discharge ductleads first downwards and then as the discharge duct approaches thedischarge end upwards in the vertical direction, so that the dischargeduct opens upwards to a free air space and to a normal air pressure. 4.The apparatus of claim 3 wherein the discharge duct leads through a wallof a stock and/or wire water second tank and the discharge duct opensupwards inside the second tank and wherein the second tank has anopening to normal air pressure.
 5. The apparatus of claim 4 wherein theoverflow surface for wire water flowing through the discharge duct endof the discharge duct opens in the second tank in a space above a liquidsurface in the second tank.
 6. The apparatus of claim 1 wherein the endof the discharge duct is connected to a second tank and the second tankhas a tank overflow for the wire water, the tank overflow forming thedischarge duct discharge end overflow.
 7. The apparatus of claim 1wherein the discharge end of the discharge duct is arranged to leadinside a second tank, the discharge end of the discharge duct formingthe overflow and the overflow surface, and the second tank opens to thefree air space, and wherein the second tank has a separate overflow, andwherein the second tank has a duct leading to a pump and from the pumpto the deaeration tank, and that said duct allows wire water to bepumped into the deaeration tank.
 8. The apparatus of claim 1 wherein thefirst branch duct is connected to a suction side of a headbox feed pump,and from the headbox feed pump to communicate with the headbox of thepaper machine.
 9. The apparatus of claim 8 wherein the discharge duct isconnected to a second branch duct, the second branch duct beingconnected to a source of virgin stock so that virgin stock can be addedto wire water from the discharge duct and the second branch duct beingconnected to a suction side of a second pump and the second pumpconnects to a centrifugal cleaning plant and the centrifugal cleaningplant connects to the first branch duct which communicates with theheadbox through the headbox feed pump.
 10. The apparatus of claim 1wherein the discharge duct has a branch duct which communicates with theheadbox at a plurality of different locations in the width direction ofthe headbox in order to allow dilution of the stock passing to theheadbox.
 11. The apparatus of claim 1 further comprising acyclone-shaped device in which a centrifugal field is produced, thecyclone-shaped device being positioned in the inlet duct between the atleast one source of wire water and the deaeration tank, to separate airin the form of bubbles from wire water.
 12. The apparatus of claim 11further comprising an air separation duct connected to thecyclone-shaped device, the air separation duct for containing air beingseparated from the inlet duct.
 13. The apparatus of claim 12 wherein theoverflow surface of the overflow of the discharge duct is located 5-10meters below the defined wire water surface within the deaeration tank.14. The apparatus of claim 1 wherein the first branch duct is incommunication with the headbox through a pump and a separator.
 15. Amethod of deaerating wire water and passing the deaerated wire water toa headbox with reduced pressure variation the method comprising thesteps of: collecting wire water from at least one source of wire waterand conducting the wire water to a deaeration tank on which a vacuum ispulled, the wire water defining a surface within the deaeration tank;draining wire water from the deaeration tank from below the wire watersurface in the deaeration tank through a discharge duct; maintaining anoutlet of the discharge duct at a constant pressure with an overflow,the overflow defining a surface which is below the wire water surfacewithin the deaeration tank; and conducting a portion of the wire waterdraining through the discharge duct by a branch of said discharge ductto communicate with the headbox.
 16. The method of claim 15 wherein theoverflow defines a surface which is 5 to 10 meters below the wire watersurface within the deaeration tank.
 17. The method of claim 15 wherein apump draws the portion of the wire water which is conducted to the headbox from the discharge duct, and also draws stock having a higher fibercontent than the wire water, so that the portion of the wire water ismixed with the stock having a higher fiber content and communicated tothe head box.
 18. The method of claim 15 wherein a portion of the wirewater which is drained from the deaeration tank through the dischargeduct is lead through a wall of a stock and/or wire water tank, thedischarge duct opening upwardly inside the tank, and wherein the tankhas an opening so as to form a free air space.
 19. The method of claim15 wherein the discharge duct has an end which forms the outlet of theduct and the overflow.
 20. The method of claim 15 wherein the end of thedischarge duct is connected to a second tank and wherein the second tankis arranged to create the overflow and the defined overflow surface. 21.The method of claim 15 wherein the discharge duct is arranged to leadinside a second tank, the discharge end of the discharge duct formingthe overflow and the overflow surface, the second tank opening to thefree air space and further comprising the step of pumping water from thesecond tank to the deaeration tank.
 22. The method of claim 15 furthercomprising the step of passing wire water flowing through the inlet ductthrough a cyclone and separating air from wire water flowing through theinlet duct.
 23. The method of claim 15 wherein the portion of the wirewater draining through the discharge duct by the branch of saiddischarge duct communicates with the headbox through a pump and aseparator.